/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode* res = nullptr;
        ListNode* ptr = nullptr;
        int carry = 0;  // 进位
        while (l1 || l2) {
            int n1 = l1 ? l1->val: 0;
            int n2 = l2 ? l2->val: 0;
            int sum = n1 + n2 + carry;
            if (!res) {
                res = ptr = new ListNode(sum % 10);
            } else {
                ptr->next = new ListNode(sum % 10);
                ptr = ptr->next;
            }
            carry = sum / 10;
            if (l1)
                l1 = l1->next;
            if (l2)
                l2 = l2->next; 
        }
        if (carry > 0)
            ptr->next = new ListNode(carry);
        return res;
    }
};